A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to control the lithographic process to place device features accurately on the substrate, alignment targets are generally provided on the substrate, and the lithographic apparatus includes one or more alignment sensors by which positions of targets on a substrate can be measured accurately. These alignment sensors are effectively position measuring apparatuses. Different types of targets and different types of alignment sensors are known from different times and different manufacturers. A type of sensor widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Pat. No. 6,961,116 (den Boef et al). Generally targets are measured separately to obtain X- and Y-positions. However, combined X- and Y-measurement can be performed using the techniques described in published patent application US 2009/195768 A (Bijnen et al). The contents of both of these applications are incorporated herein by reference.
Advanced alignment techniques using a commercial alignment sensor are described by Jeroen Huijbregtse et al in “Overlay Performance with Advanced ATHENA™ Alignment Strategies”, Metrology, Inspection, and Process Control for Microlithography XVII, Daniel J. Herr, Editor, Proceedings of SPIE Vol. 5038 (2003). These strategies can be extended and applied commercially in sensors of the type described by US'116 and US'768, mentioned above. A feature of the commercial sensors is that they measure positions using several wavelengths (colors) and polarizations of radiation (light) on the same target grating or gratings. No single color is ideal for measuring in all situations, so the commercial system selects from a number of signals, which one provides the most reliable position information. Alternatively the system can also use a linear combination of signals from different colors and/or polarizations, as described in a patent application comprising the present applicant's internal file number 2014D00195, which is herein incorporated by reference.
There is continually a need to provide more accurate position measurements, especially to control the overlay error as product features get smaller and smaller. One cause of error in alignment is as a result of the alignment target being exposed with a different shift to nominal than the surrounding product features, due to the large difference in pitch between the alignment target and product features. To explain, alignment targets are generally formed of gratings with features far larger than the features of the device pattern which is to be applied to the substrate in the lithographic apparatus. The required positioning accuracy is therefore obtained not by the fineness of the alignment grating, but rather by the fact that it provides a periodic signal that can be measured over many periods, to obtain overall a very accurate position measurement. On the other hand, a coarse grating is not representative of the actual product features, and therefore its formation is subject to different processing effects than the real product features.
The alignment targets are typically applied to the substrate throughout a device manufacturing process, using a lithographic apparatus similar or even identical to the one which will apply the patterns for subsequent product layers. The product features become subject to slightly different errors in their positioning than the coarser alignment grating features, for example due to aberrations in an optical projection system used to apply the pattern. The effect of this in current alignment sensors is that the measured position contains unknown errors, being neither the position of the coarse grating nor that of the product features.
To address this error mismatch between coarser alignment grating features and product features (referred to herein as “mark print error”), an alignment target has been developed which allows the mark print error to be measured, and therefore corrected for. Such alignment targets may be referred to as differential sub-segmented targets (DSM targets) and are described in WO2014/014906 which is herein incorporated by reference. While such DSM targets are effective for measuring the mark print error, their effectiveness has been found to be compromised by some substrate processing steps. These processing steps, of which one is etching, can cause asymmetrical deformation of the target which degrades performance of the target. This limits their usefulness in normal substrate alignment sequences, as the targets are typically measured subsequent to this processing.